Most people are familiar with radio communications systems in which a single transmitting site transmits to an associated coverage area. Radio and television broadcasters use this approach. Due to power output and other limitations, the coverage area of a single transmitter may be too limited to reach the desired audience of radio users. In radio and television broadcasting, this problem is sometimes solved by providing a "network" of multiple transmitting stations all carrying the same program. To avoid interference, nearby stations operate at different frequencies (i.e., on different radio or television "channels"). A person driving from say, Washington D.C. to Richmond, Va., may for example, retune her radio from the Washington network-affiliated station to the Richmond network-affiliated station when the Washington station becomes too weak to hear. "Cellular" radio-telephone systems operate in a similar way by automatically controlling the user's cellular phone to automatically retune to a different frequency as the user leaves one "cell" (transmit site coverage area) and enters another--while automatically routing the user's call signals to the new "cell" and transmit frequency.
The types of radio services described above are carefully designed to avoid interference between radio transmitters operating on the same frequency. For example, the Federal Government only licenses a single radio or television to operate on any given frequency in a major metropolitan area, and typically requires substantial frequency spacing between nearby transmitters to make sure their transmissions do not interfere.
Simulcast systems transmit substantially the same signals simultaneously from multiple physically-separated transmitters to achieve a wider coverage area than could be accomplished using a single transmitter. Unlike many other systems, however, each of the simulcast transmitters transmits the same signals on substantially the same frequency at substantially the same time, so that radio receivers within intentionally overlapping coverage areas can receive signals from multiple simulcast transmitters simultaneously without interference.
FIG. 1 shows a simple example of a simulcast transmission system comprising three physically-separated transmitter sites S1, S2 and S3. A common control point C sends each of these transmitter sites S1, S2 and S3 the same signal for transmission. Transmitter site S1 transmits the signal over a particular radio frequency to its coverage area A1, and transmitter sites S2 and S3 each transmit this same signal at substantially the same time over substantially the same radio frequency to their respective coverage areas A2 and A3. A mobile radio receiver M can receive the simulcast transmitted signal so long as it is within at least one of the coverage areas.
The system is designed so that coverage areas A1, A2 and A3 intentionally overlap one another (see the cross-hatched regions in FIG. 1) to eliminate "holes" in the overall system coverage. The radio receiver M may receive the transmissions from more than one site whenever it is in one of these overlap regions. For example, if receiver M is within the overlap region marked "X", it is within both the A2 coverage area of transmitter site S2 and the A3 coverage area of transmitter site S3--and will receive both the signal transmitted by transmitter S2 and the signal transmitted by transmitter S3. The receiver M will typically be "captured" by the strongest received signal (at least if FM modulation is being used), and a weaker one will have little or no effect on reception. However, if the receiver M is positioned in the overlap area so that it receives each of the multiple signals at about the same strength, both signals will contribute to what is received by the radio receiver. These multiple received signals will not interfere with one another only if they are at nearly or exactly the same frequency and have nearly or exactly the same timing.
The timing aspect is especially critical. Even small timing differences (e.g., on the order of thousandths of a second) can cause problems in reception clarity and reliability. For example, even small timing differences can garble high speed digital signals, causing the receiver to miss important calls.
There are problems in providing precise timing synchronization as described above. For example, because the transmitter sites S1, S2 and S3 are physically separated from one another, they are each connected to control point C by a different communication link. Link L1 connects site S1 to control point C, link L2 connects site S2 to the control point, and link L3 connects site S3 to the control point. In the general case, links L1, L2 and L3 have different lengths and other characteristics that cause the delay time it takes for signals to travel over the links to be different. Therefore, the time delay involved in transmitting the transmission signal from control point C to transmitter site S1 over link L1 will, in general, be different from the time delay involved in transmitting the common signal to site S2 over communication link L2--and the link L3 used to communicate the signal to transmitter S3 will, in general, provide a still different time delay. These different time delays must be compensated for if the simulcast system is to operate reliably to provide synchronized transmit timing.
Much work has been done in the past in an attempt to solve this problem. One prior system, described in commonly-assigned U.S. Pat. No. 5,172,396 filed Dec. 27, 1999 to Rose et al., uses a "master" resynchronization circuit at the control point C to generate reference timing. These reference tones are sent to each of the transmit sites S1, S2, S3. Each transmit site has a resynchronization ("resync") circuit that takes data received over the respective control point link and resynchronizes it by aligning it with the reference timing. In this prior system, the reference timing is encoded in tones including a lower frequency (300 Hz) "gating" signal and a higher (2400 Hz) frequency reference signal. The reference tones are typically sent over high-quality, extremely stable signal paths since any variation or noise can effect system performance.
A further improvement described in commonly-assigned copending U.S. patent application Ser. No. 08/364,467 involves placing a Global Positioning System ("GPS") receiver at each simulcast transmitter site. Such GPS receivers are now commonly used for navigation and other purposes. The GPS employs 24 satellites in 55.degree. inclined orbits 10,000 miles above the Earth to transmit precise timing signals that allow a GPS receiver anywhere on Earth to determine its own location. A 1575 MHz transmission carries a 1-MHz-bandwidth phase-modulated signal called the clear acquisition (C/A) code. When a GPS receiver receives this signal from at least three GPS satellites, it can determine its own latitude and longitude to an accuracy of about 30 meters.
In this prior simulcast system, the control point C does not need to distribute reference edges/tones. Instead, the GPS receiver at each site is used to provide a stable, precise timing reference (e.g., a precise, stable 9600 bps clock and a lower frequency gating signal). Each transmitter site S1, S2, S3 resynchronizes its received data using the timing references provided by its local GPS receiver. Because these reference timing signals are not sent over the same type of links used for signals to be transmitted, there is no need to provide wide band, stable channels. Moreover, any link latency variation (within a gating window) is automatically corrected when a "resync" is performed.
Although the GPS arrangement described above has been highly successful, further improvements are possible. One previously unsolved problem was that the amount of time delay compensation possible was too limited and was dependent on the period of the reference signal. Ericsson's EDACS land-mobile trunked radio communications system, as an example, constrains the choice of a reference gating frequency to be a multiple of the frame timing (30 Hz) and a sub-multiple of the data transmission rate (9600 bps). A 300 Hz frequency has been used in commercially released EDACS systems, limiting the amount of time delay compensation to the period of this reference frequency (i.e., 3.3 milliseconds for a 300 Hz reference). Compensation greater than this amount would lead to ambiguity in the amount of correction required. Selecting a lower reference frequency (e.g., 60 Hz) provides a longer gating period (e.g., 16.6 milliseconds), but even this period may not provide enough compensation range depending upon the particular installation involved--and the EDACS system constraints discussed above do not allow the reference frequency to be arbitrarily chosen based on the amount of compensation required.
Another previously unsolved problem was that prior compensation arrangements were sometimes overly complicated. For example, in one prior EDACS arrangement, a different delay circuit was provided for each voice path and for each data path. This compensation approach did not take advantage of the fact that the voice and data paths can be sent over the T1 link in a common stream, and required the use of redundant circuitry that increased system cost.
The present invention solves these problems. It provides improved resynchronization arrangements and techniques for automatic, dynamic correction/compensation of path delay changes that provides a timing correction range that is independent of the gating reference frequency. Techniques and arrangements provided by this invention can correct delays over a wide timing range not known ahead of time, dynamically correct for path delay changes during system operation without loss of service, and apply a single delay correction to an entire transmitter site. These techniques and arrangements may provide path delay change correction up to a maximum limited only by the transmit system protocol--for example, up to one second. They also eliminate the need to be concerned with the phase of the gate reference from a GPS receiver, can be used to eliminate resynchronization at the control point, and are fully compatible with other improvements such as "auto align clear voice" and land line backup features disclosed in commonly assigned copending patent application Ser. No. 08/535,932, filed Sep. 28, 1995.
In accordance with one aspect provided in accordance with the present invention, the simulcast control point uses a GPS receiver to provide a reference signal. The control point provides this reference signal along with voice and data signals for transmission (plus additional timing and control signaling) to multiple simulcast transmit sites over associated communication links. Each transmit site includes a GPS receiver that provides the same frequency reference signal. A timing comparator compares the timing of the reference signal generated by the transmit site's local GPS receiver with the timing of the reference signal the transmit site receives from the control point over its control point communications link. The result of this comparison is used to adjust a variable delay added to the control point communication link's inherent delay.
If the communications link is a T1 or E1 microwave link or other TDM link, this additional variable delay may act to delay the composite TDM data stream before a subsequent multiplexer separates the data stream into its individual signal components. Thus, at the transmit sites, all of the signals--data, voice, and reference--are delayed by the same variable added delay before being separated and sent to the individual RF channel repeaters.
In one example arrangement, the control site generates and provides, over the control site communications link, a gating reference signal in addition to the reference signal. The transmit site extracts the reference signal from a land-line composite signal and compares it to the locally generated GPS reference. A timing comparator at the transmit site adjusts the variable delay to force the two reference signals to "match up." Since each transmit site does the same thing, each transmit site succeeds in adjusting the gating reference to be "the same" as at all other transmit sites.
The GPS receivers at each transmit site in this example are set up with a fixed delay relative to the GPS receiver at the control point (plus or minus any optional site specific desired offset) that forces all sites to "wait" by this amount. The variable delay at each site absorbs any delay not used by the link. Thus, this fixed delay--not a gating reference frequency--determines the correction range for the overall system. The fixed delay can be made as large as convenient (e.g., one second).
Because the overall system is monitored and corrected based on the GPS reference signal (which may be one pulse-per-second for example), it is very quick in responding. The reference signal comparison is part of the normal system operation, so there is no need to operate in any different "mode" or otherwise discontinue normal system operation to dynamically compensate for changes in variable path delay. The delay adjustment technique is completely transparent, continuous, and operates alongside normal system operations. Unlike some prior techniques, no special "alignment" or "synchronization" mode needs to be executed to adjust the delay.